Add scene side discrete level of detail.
New node `LOD` for UI, and `LODManager` within `World` for automatically updating child visibilities based on distance from cameras.
* Added additional information to the camera documentation to explain
how the 'current' attribute behaves when multiple cameras are in a
scene.
(cherry picked from commit 349c750b18acf770ef6f0e43643c8a37a3beb450)
This mentions that internally, the inverse camera projection is used
to perform projections.
(cherry picked from commit fbec10179ad2a058a5f6d2689dba9db813bdc83f)
This allows for lower field of view (or higher zoom) in orthogonal
and frustum camera modes.
The property hint also allows setting the size with greater precision.